The usefulness of thin single crystal silicon films on electrically insulating substrates for electronic devices, such as integrated circuits, becomes apparent when one considers that these circuits are contained within the top micrometer of the silicon material. In addition to improving the electrical circuit characteristics, the material is used more effectively. From the device standpoint higher circuit densities as well as improved device performance are obtainable in the area of high speed signal processing, lower power consumption, and higher tolerance in radiation environments.
Heating is the principle parameter used in controlling the crystallization of bulk and thin film semiconductor silicon. However, a thin film need only be locally heated, as dictated by its thickness and as such differs in its requirements of heat application from that of the bulk material. For heating purposes, lasers are unique in that their beams can be projected over long distances, are collimated, and can be controlled spatially as well as temporally. Laser beams can irradiate the top layer of a material leaving other regions essentially unheated. With all these features laser beams have the potential of being the principal heat source used in the crystallization of thin silicon films.
Crystal growing from a single component melt and its subsequent uncontrolled freezing, results in a polycrystalline structure. It has been shown that for seeded crystal growth, freezing liquid-solid interfaces that are concave towards the liquid favor single crystal growth. An important aspect in solid state devices is the crystalline quality of the semiconductor material as it effects the device's electrical characteristics. Until the present invention, however, the mechanism that can be used to ensure single crystal growth upon freezing of a melted semiconductor such as silicon has not been recognized or understood. For a more thorough and detailed discussion and examination of prior art techniques and developments in this area, reference is made to Appendix 1 to this patent application entitled "THE CRYSTALLIZATION OF THIN SILICON FILMS WITH LASER SHAPED HOT ZONES", June 1985, authored by the present inventor and hereby incorporated by reference in its entirety.